This proposal is for a Program Project Grant consisting of three Projects and three Cores. Its overall goal will be to evaluate the limits of adaptive changes in the relationship between spinal circuits and the musculoskeletal system when this interaction is disrupted. In each of the projects this disruption will be studied in model systems using transection and surgical repair of peripheral nerves. In Project I (English), the capacity for spontaneous adaptation, the effects of treadmill training, and operant conditioning of spinal reflexes on the outputs of spinal circuits will be studied after sciatic nerve transection in rats. The effectiveness of combining these approaches with protocols known to enhance the early regeneration of axons in injured peripheral nerves will be determined. In Project II (Gregor), the roles of short term compensatory strategies on longer term adaptive changes will be studied after transection and surgical repair of a single muscle nerve in cats. Using kinetic, kinematic, and EMG analyses, the extent of compensation by connective tissues, the effects of enhancing axon regeneration, and the roles played by cutaneous sensory systems in these changes will be studied. In Project III (Nichols), the roles played by different forms of proprioceptive feedback in the functional deficit following surgical reinnervation of the main cat plantar flexor muscles will be studied. Using acute and chronic experiments in conjunction with biomechanical modeling, the mechanisms underlying both short term compensation and longer term adaptation will be elucidated. Using intense treadmill ramp training during the recovery period, the preservation of normal reflexes will be studied. This PPG brings together a team of established scientists from diverse backgrounds, with a common goal to continue to strengthen the science base underlying clinical rehabilitation.